From: Gudguyham@aol.com
Sent: Monday, July 23, 2012 5:00 AM
To: jim.thom@telus.net ; amps@contesting.com
Subject: Re: [Amps] Drake L4B 50K Resistors HOT!!
In a message dated 7/23/2012 5:53:55 A.M. Eastern Daylight Time,
jim.thom@telus.net writes:
was just repairing a Drake L4B that I picked up from my want listing on
QTH.com. I just happened to measure the temperature of the 50K 50 watt
resistors in the power supply with my infrared thermometer and WOW!!
The middle of the resistors run 510 degrees f !! I could not believe it!
Needless to say there outta here! I won?t need them anyway. I?m using the amp
to prototype my QSK board for the Drake L4-L7 amplifiers that will use resistor
cutoff bias instead of the +120 volts that the resistors provide.
73
Jim W7RY
OH SO TRUE!!!!!!! it cracks me up. Swan built the Mark I and then the Mark
II. Then they revised the
Mark II. Heathkit built the Warrior and then the SB series came in. When
Collins decided to raise the grids above ground with caps and use masses
amounts of cut off bias voltage the other companies followed suit with raised
grids above ground and high cut off voltage. The Swan Mark II revision (second
edition) had the raised grids and higher voltage cut off . They completely
forgot or second guessed what they learned from their previous amps. Heathkit
followed suit with the SB-200 and 220 with raised grids and masses amounts of
cut off bias. Drake came on the scene at the time with their L4 and later L4B
they started right off the bat with that nonsense. Now poor Jim (and the rest
of us should) go in their and do what Jim has done. The problem is when you do
that it marries the power supply to that amp! It seems as though Collins had a
stability problems with the 30L1 without enough cut off bias the 4 811A tubes
could take off so once you plaster 120vdc on them they aint goin' anywhere in
cutoff. The Yeasu FL-2100B has same problem so in the Z model they put in a V
tripler so add more V for cut off. Even when Viewstar caught wind of this they
did it also. It seemed to be the status quo to follow what Collins did
because he was trying to stabilize the 30L1 which the 3-500 tube amps didn't
need. Oh, not to Mention the Kenwood TL-922 did the same thing as well. The
only one out there that kept the faith was Denny. he didn't fall for that
nonsense he just had a different set of problems.
### I forgot to mention that I also tossed ALL the grid to chassis caps and
ALL the grid to chassis chokes. Replaced with wide CU strap. Drive
requirements dropped 20-25 watts in all cases, and all bands.
Then the xcvr po is reduced by 20-25 watts pep. Then the xcvr’s IMD is a
lot less . Cleaner signal into the amp means a cleaner, lower imd output from
the amp.
If u want a grid fuse, simply wire a 3agc fuse holder on the rear apron..and
wire the FAST grid fuse between chassis and lower end of grid shunt. If grid
fuse pops open, no path for dc grid current, and amp shuts down.
No PO... and input ser rises to infinity. Xcvr then shuts itself off. Use
the vacated spaces of the pair of oem 50 k 50 w ww..and install a 50 ohm @
50 watt WW..and that is ur new glitch R. install a HV fuse in series with the
glitch R... and now its fully protected. Whoa.... the drake PS already uses a
.82 ohm tiny resistor as a HV fuse in the B+ lead..and it will work.
later... Jim VE7RF
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